This invention relates to methods and apparatus for sputtering and in particular, but not exclusively, to sputtering material onto workpieces having high aspect ratio formations.
As is well known in the art, the sputtering process normally consists of causing energetic ions from a plasma to strike a target of the desired material, or a component part thereof, so that atoms from the target are ejected. In sputter deposition these ejected atoms are intended to be deposited on the surface of a workpiece. The initial direction of travel of the atoms is close to being cosine, however several apparently conflicting factors can determine how many atoms reach the workpiece and with what level of uniformity across the surface of the workpiece at what angular distribution and to what extent they penetrate into high aspect formations, occurs.
Thus, for example, given a uniform erosion of a large target (as is typically the case) a low chamber pressure will usually produce good uniformity, across a substantially planar surface. This is because of the relatively low working gas density reducing the number of scattering collisions. For those atoms which are ejected substantially normally to the target, there is a low probability of collision and hence a higher probability that they will reach the workpiece with the same angular distribution. Conversely, high pressures produce a higher number of collisions causing more of the sputtered atoms to be lost to the chamber walls. This tends to lead to a deposited layer that is thicker towards the centre than the outside of the workpiece. However at higher pressures, with the relatively high probability of collisions of sputtered material, there is a higher probability of sputtered material being ionized due to these collisions. Ionised sputtered material may then be attracted normally to the substrate surface by the application of a negative charge e.g. through the use of a bias R.F. or D.C. power supply or self biassing controlled by a variable impedance to ground potential. The need to have a good percentage of the ejected atoms ionised is particularly great, when there are high aspect ratio formations in the workpiece, because it is only those atoms which are travelling at or near perpendicularly to the opening of the mouth of the formation. As the aspect ratio increases e.g. as a recess diameter decreases and/or depth increases the direction of the arriving material needs to be more nearly perpendicular. This is to avoid the recess being closed off at its mouth by sputtered material arriving at non-normal angles and to ensure an adequate coverage of the base of recesses. Hence, an ionised high pressure sputter process will achieve improved base coverage in high aspect ratio recesses, but at the expense of bulk surface uniformity across the workpiece.
Additionally the Applicants have also determined that where, particularly, an internal or immersed R.F. coil is used lack of uniformity arises through non-symmetrical coupling at the feedthroughs to the R.F. coil. These effects increase significantly with chamber pressure. This invention achieves improved base coverage previously achieved by high pressure ionisation processes whilst simultaneously achieving good surface uniformity previously achieved by lower pressure processes without the non uniformities inherent in internal R.F. coil arrangements of the prior art.